Voltage excursions are frequently encountered which may damage or destroy sensitive electronic equipment and components. Examples of these voltage excursions are, but not limited to, electrical surges, voltage transients, over voltages, lightning energy, induced voltages, current surges, switching transients, etc. These will be referred to generally as electrical surges.
In a railroad installation which contains electrical equipment, such as a signal bungalow or case, it is a common practice to protect critical electrical control circuits such as those controlling the track signals from electrical surges. These surges may be attributable to lightning or other known or unanticipated sources which create an unusually high voltage on the circuit. Surge protection is important because high voltage surges may destroy electrical components or contribute excessive electrical noise to a circuit causing it to malfunction or otherwise perform unpredictably.
To prevent damage from electrical surges, over-voltage or lightning arrestors are used to couple the energy from the circuit to ground when an electrical voltage surge is detected on the circuit exceeding a predetermined voltage with respect to a reference point. This effectively bleeds the energy of the electrical surge to ground and protects the sensitive electrical circuit components. Currently, when it is necessary to protect more than one circuit, a surge arresting device is coupled to each circuit and attached to a ground bus. There may be several devices employed in this manner. The ground bus is then connected to an outside reference ground.
While these configurations have provided some protection, it has been found that due to impedances of the ground lead some of the arrestors (particularly those farthest from the connection to the reference ground) would see a higher impedance.
Consequently, instead of an arrester coupling most of the energy of the electrical surge to ground, the line potential will be much higher due to the inductance of the ground lead. When additional arresters are used the voltage difference between two lines will be mainly determined by the difference in the impedance of their ground leads. If this difference is large, the potential voltage between the two lines may result in damage to or malfunctioning of sensitive electronic equipment and components.
The deleterious effects of electrical voltage surges are especially troublesome at high frequencies. This is because the impedance of the ground path is a function of not only resistance, but also inductance and capacitance. At relatively low frequencies, such as 60 hertz, the impedance is sometimes due primarily to the dc resistance. Conversely, at high frequencies, such as 30 megahertz, the inherent inductance and capacitance of the ground path may elevate the impedance to such a degree that the path is essentially open. In this case the impedance to ground apparent to the arrestor might be so high that it would not couple the electrical surge to ground and instead it would be conducted to the circuit sought to be protected.
It is desirable to provide a surge arrestor assembly in which all the arrestors in the assembly see the same low impedance to ground, and wherein the assembly is effective at high frequencies as well as at low frequencies.